In a moving magnet type exciting linear motor, a mover is a magnet and a driving electric wire is provided on the stator side. In this arrangement, as the wire under no stress is not broken and heat generation is made on the stator side, cooling can be easily made by coolant or the like. Accordingly, this linear motor is used in various fields including a semiconductor fabrication apparatus requiring high reliability and accuracy.
In a semiconductor exposure apparatus, for the sake of improvement in throughput, it is necessary to increase the size of a substrate (wafer) on which the pattern of a reticle is transferred, and to improve an acceleration and maximum speed of the linear motor. From this background, as a longer stroke in comparison with the size of the mover is required, plural coil pairs are must be provided. If all the coil pairs are connected to the same current driver, a thrust can be generated, however, a coil outside of a magnetic field of the mover does not produce a thrust, but only causes a loss (heat generation) due to electric resistance of the coil. Thus the efficiency of the motor is extremely low.
To prevent this inconvenience, current drivers may be connected to the coil pairs of respective phases, such that a current command value of each current driver can be independently supplied to each connected coil pair, and a current cannot be fed through a coil outside of the magnetic field of the mover. This method requires current drivers and command value generation units corresponding to the number of the coil pairs. As the cost increases in proportion to the stroke, this method cannot be employed in the case of the long stroke.
As another method, the current drivers may be connected to the respective coils via relays such that a current is selectively fed through coils within the range of the magnetic field of the mover (See Patent Document 1: Japanese Published Unexamined Patent Application No. Hei 6-284785 and Patent Document 2: Japanese Published Unexamined Patent Application No. Hei 8-111998).
FIG. 3 illustrates the arrangement of a conventional linear motor driving circuit using relays. In FIG. 3, alphabet M denotes a mover magnet; reference numerals CAn−1, CAn and CAn+1, an A-phase stator coil array in the linear motor; CBn and CBn+1, a B-phase stator coil array in a linear motor; SAn−1, SAn and SAn+1, switches to the A-phase coils; SBn and SBn+1, switches to the B-phase coils; U, an absolute type ultrasonic sensor; DA, an A-phase driver; DB, a B-phase driver; and G, a controller. The interval between same-phase coils is 1.5 times of the period of the magnetic field of the mover magnet M.
Next, a driving method for moving the mover magnet M in a +x direction (leftward direction in FIG. 3) will be described. As to energizing of the stator coil, assuming that the mover magnet is positioned as shown in FIG. 3, to move the magnet in the +x direction, energizing of the A-phase is changed from the coil CAn to the coil An+1, and the controller G feeds a 2-phase drive current at a phase angle corresponding to the position.
In accordance with coil winding and connection of coil to the driver, upon change of energization to the same-phase coils (e.g., from the coil CAn to the coil CAn+1), a command current to the driver must be inverted. However, in this case, as disclosed in Patent Document 3(Japanese Published Unexamined Patent Application No. Hei 11-341853), the control accuracy of the linear motor is degraded, and the load on the driver is increased. Accordingly, as disclosed in the Patent Document 3, it is preferable to connect the same-phase coils to the same-phase driver such that the polarity is inverted by each of same-phase coils. In such connection, in the position in FIG. 3, the drive current command value at a phase angle of 180° is supplied to the coil CAn+1 and the drive current command value at a phase angle of 90° is supplied to the coil CBn.
In an exposure apparatus requiring accuracy on the order of nanometer and a several hundred mm moving stroke, a laser interferometer is often used for position measurement. This high-accuracy and long-stroke measurement means generally is not an absolute type means but an incremental type measurement means to detect relative position information. Accordingly, to obtain absolute position information, position data must be initialized (calibrated). Generally, as the initialization of position data, the mover is moved to a particular position (a thrusted position, attachment position of a photo interruptive switch for initial position detection, or the like) (this movement is referred to as initialization driving), and the measurement value is rewritten with a desired value there. However, in this initialization driving, the incremental type sensor including an interferometer cannot be employed. Patent Document 4(Japanese Published Unexamined Patent Application No. Hei 11-316607) discloses initialization of an interferometer for initialization driving using an absolute sensor with accuracy lower than an incremental sensor such as a ultrasonic sensor.